The Aug. 17, 1999 Turkey earthquake (BDSN surface wave magnitude Ms=7.8)
took place along a segment of the North Anatolian
fault which had been a seismic gap of over 100 km
between the two earthquake ruptures of 1963 (M6.3),
and 1967 (M7.2) [Toksöz et al., 1979]. The automated
Centroid Moment Tensor
(CMT) solution determined for this event by the Harvard group shows an almost
pure right lateral strike slip event (strike=268o, dip=84o,
and slip=180o), with a source depth of 16 km. Three months later another large event
(BDSN: Ms=7.4; Harvard CMT solution: strike=265o, dip=65o,
and slip=-158o) occurred 100 km east of the August epicenter.
The North Anatolian fault has a rich history of repeating earthquakes and is known
to involve progressive failure of adjacent segments.
Understanding the triggering mechanism of adjacent segments of fault
requires knowledge of the slip distribution of previous events. We
conducted a preliminary analysis of the distributed fault slip
and an estimate of source rupture process, based on teleseismic
observations, that allows varying mechanisms.
In the analysis, we used bodywave displacement seismograms recorded by the Incorporated
Research Institutions for Seismology (IRIS)/ Global Seismographic
Network (GSN) stations at teleseismic distances.

A kinematic model of the distributed fault rupture was obtained using the
method of Antolik [1998], and Antolik et al. [1999].
This method inverts seismic moment rate functions that may be derived
by either empirical Green's function deconvolution, from a smaller collocated event with the same radiation characteristics, or by the deconvolution of theoretical Green's function,
for an appropriate source radiation pattern and depth.
In either case the deconvolution serves to remove path effects, leaving an estimate
of the seismic moment rate. For the Izmit earthquake, we obtained the seismic moment rate using theoretical
Green's functions computed using the reflectivity method, and
the focal parameters obtained by the Harvard group.

The inversion suggests a bilateral rupture extending 50 km to the west
and 60 km to the east of the hypocenter with its major slip occurring to the
west (Figure 14.1, 14.2). The average and peak slips are found to be 4.2 and 9.3 m, respectively.
The rupture propagated upwards, with most of the slip occurring near the surface. The
total areas of displacement is estimated to be about 1200 km2.

Figure 14.1:
Epicenters of the August 17 and November 12, 1999 earthquakes and previous
major events in this region. The focal mechanisms of the 1999 events are based
on the Harvard CMT solutions. Yellow dots indicate the
aftershock locations since Aug. 17. We also plotted the surface trace of the
grid used in the inversion (red line).

Figure 14.2:
Slip distribution obtained by inversion of the P MRFs at stations
ADK, INCN, KONO, MA2, PAB, SJG, and YSS. The main slip occurs to the west
of the epicenter, but there is evidence for a significant amount of slip
to the east, which suggests a bilateral rupture. Yellow dots indicate the
aftershock locations during the first 4 months following the main shock.
The position of the hypocenter is given by the yellow star.

Figure 14.3:
Inversion result, i.e., three subevents (E1, E2, and E3), source-time
function, and spatial distribution of three point sources in the grid on the
given fault plane.

We have also employed the inversion method of Kikuchi and Kanamori [1991] that models
the entire source process as a series of point source subevents and thus
permits the determination of subevent moment-tensors, point-source
locations, and the time-function simultaneously.

The main source process can be explained by several subevents (Figure 14.3).
The first subevent (E1) is determined with a symmetric trapezoidal
unit time function (,)=(3s, 6s)
at the NEIC
epicenter with a source depth of 10 km and a released moment
0.96 x 1020 Nm or Mw=7.2). The mechanism of this subevent
is almost a pure right
lateral strike slip ( strike=275o, dip=89o, and slip=-180o),
similar to the CMT solution. Smaller subevents, E2 and E3
((,)=(3s, 3s), and (1.5s, 1.5s)) with a reversed
strike slip fault motion and moment release of 0.3 x 1020 Nm
appear to have occurred 30 (at 12 s) to 60 km (at 34 s) east
of the first subevent. However, the determination of these smaller subevents
was not stable being affected by a small change of the input parameters
such as the unit time functions, weighting factors for the individual
waveform data etc... The trade-offs between the source parameters of smaller
subevents and structural noise seems to be also significant in this frequency
band. If the first subevent corresponds to the major slip determined to
the west of the epicenter, then the stress drop of this event
is roughly estimated to be 7.6 Mpa according to the following formula:

where S=50 x 20 km2

Discussion

Our preliminary analysis using two different approaches suggests a
bilateral propagation of the main source rupture. While the two separate
portions of the slip distribution were identified on the fault plane,
the major slip occurred to the west of the epicenter. The first subevent
determined in the source process has a moment release of 0.96 x 1020 Nm
and may correspond to the rupture of the main asperity
which has a dimension of 50 x 20 km2. Adopting the moment and slip
dimension, the stress drop of this event is estimated to be 7.6 Mpa.
The slip distribution to the east of the epicenter roughly coincides
with the location of the second subevent which was determined 12 s
after the first subevent in the source process. Due to the trade-off
between the subevent source parameters and structural noise the
determination of the smaller subevents is not stable.
On the other hand strong ground motion
seismograms recorded at several stations near the source area such as
YPT (20 km west of the epicenter) indicate two distinct events
which occurred with a time interval of 30 s. The separation of
the two events decreases in the waveforms recorded by
stations at further distances to the west, which may offer a support for the
second subevent to have occurred to the east of the epicenter. The particle
motions obtained from the strong motion data at
Yarimca (YPT) Station also indicate a bilateral propagation of
the rupture.

In general, the determination of earthquake source parameters (moment tensor,
hypocenter, and source-time function) by waveform inversion is a non-linear
and multi-dimensional problem in which trade-offs
between the parameters can be significant and complicated.
The source process determination for this event will benefit from
a careful analysis of the inversion parameters, but also from
better-constrained velocity models in different azimuths of ray path propagations.
Preliminary results are nevertheless consistent with other studies, and
encouraging.